Types of image quality in image forming apparatuses include graininess, in-plane homogeneity for image, letter quality, and color reproducibility (including color stability), and the like for one page. Now that multicolor image forming apparatuses are commonly used, it is sometimes said that the most important image quality is color reproducibility. A person has memories of expected colors (especially of human skin, blue sky, metal, etc.) based on experience, and can feel uncomfortable if a color is out of an acceptable range thereof. Such colors are called memory colors, and the reproducibility thereof is considered to be more and more important when a photograph or the like is output. In regard to not only photographed images but also document images, the demand for color reproducibility (including stability) on on-demand image forming apparatuses is growing among users in offices who feel uncomfortable with difference in color from monitors, users in the field of graphic arts who pursue color reproducibility of CG images.
In order to fulfill the users' demand for color reproducibility, Japanese Patent Laid-Open No. 2004-086013 proposes an image forming apparatus for reading a patch image (measurement image) formed on a printing medium with a color sensor provided on a printing medium conveyance path. Compared with an off-line colorimetric value measurer that reads a patch image on a printing medium externally exhausted by an image forming apparatus, the invention disclosed in Japanese Patent Laid-Open No. 2004-086013 has an advantage of its capability of automatically creating an ICC profile. “ICC” is an abbreviation of International Color Consortium. By a color management module (CMM) performing color conversion using the ICC profile, color matching can be achieved among a plurality of image forming apparatuses, or between an image forming apparatus and an image display apparatus.
However, with the invention of Japanese Patent Laid-Open No. 2004-086013, a problem of a phenomenon called “thermochromism”, in which chromaticity of a measurement image to be a measurement target object changes due to temperature, is caused because the color sensor is arranged on the conveyance path in the vicinity of a fixing device. It is a phenomenon caused for the reason that a molecular structure that forms a color agent such as toner or ink changes due to “heat”, or the like.
To measure a colorimetric value of a measurement image inside an image forming apparatus, the apparatus needs to be in a state after the color agent is put on a printing medium and color mixture is finished. With an image forming apparatus that uses ink as a color agent, it is necessary to measure the colorimetric value after heating and drying the ink with a dryer. With an image forming apparatus that uses toner as a color agent, it is necessary to measure the colorimetric value after mixing colors by heating and melting the toner with a fixing device. This is because a user, who demands for color matching accuracy or color stability, usually makes a judgment based on an image in a normal temperature environment. Accordingly, the color sensor needs to be arranged on a downstream side of the dryer or the fixing device in a sheet conveyance direction.
Meanwhile, to compactly configure an image forming apparatus, a length of a conveyance path from the dryer or the fixing device to the color sensor needs to be a minimum length. Accordingly, the sheet and the color agent heated by the dryer or the fixing device are conveyed to the color sensor without being cooled down to normal temperature. Further, the temperature of a printing medium becomes higher than normal temperature also because of rising temperature of internal components of the image forming apparatus, such as a printing medium conveyance guide, or the internal atmosphere.
As described above, in some cases an image forming apparatus internally provided with a color sensor is due to influence of thermochromism, and obtains a colorimetric value measurement result different from the chromaticity in a normal environment (in a normal temperature environment). There are also the cases where it is impossible to fulfill the color matching accuracy standard, which is an index of the color matching accuracy and color stability, and the reproducibility standard, which is a standard of stability, according to ISO 12647-7.